Plural modulation and demodulation circuits



Mar. 6, 1923.

L. ESPENSCHIED. PLURAL MODULATION AND nEMoDULATloN clRcU-ls.

l SHEETS-SHEET l.

ORIGINAL FILED SEPT. 30| |919.

Mar. 6, 1923. L. ESPENSCHIED.

PLURAL MODULATION AND DEMODULATION CIRCUITS.

ORIGINAL FILED SEPT. 3o, I9I9. 4 SHEETS-SHEET 2.

I N VEN TOR.

ATTORNEY Mal. 6, 1923. 1,447,204.

L. ESPENSCHLED.

PLURAL MODULATION AND DEMODULATION CIRCUITS.

ORIGINAL FILED SEPT. 30, 1919. 4 SHEETS-SHEET 3..

f" fg* 44, 0004/ 6 INVENToR.

Mar. 6, 1923, 1,447,204.

l L. ESPENSCHIED.

PLURAL MonULATloN AND DEMoDULATloN cmcuns.

ORIGINAL FILED SEPT. 30, 1919- 4 SHEETS-SHEET 4.

Dg (Owl (om INVFNTOR ATTORNEY Patented Mar.*6, 1923.- -v

UNITED STATES.

PATENT OFFICE.

LLOYD ESPENSCHIED, OF QUEENS, NEW YORK, ASSIGNOR TO AMZERICAN TELEPHONE AND TELES-MPH COMPANY, A CORPORATION OF NEW YORK.

PLURAL MODULATION AND DEMODULATION CIRCUITS.

Application ined september 30,' 1919, serial No. 327,503. Renewed December 19, 1921. serial No. 523,590.

To all lwhom t may concern.'

Be it known that I, LLOYD EsPENscHIED, residing at Queens, in the county of Queens and State of New York, have invented certain Improvements in Plural Modulation and Demodulation Circuits, of which the following is a specification.

lThis invention relates to the transmission of signals and more particularly to systems for the multiplex transmission of lradio signals.

One of the features of the invention residester appearing, are realized in a method of.v operation whereby multiplex channels are collected at the sending station and separated at the receiving station at frequencies which vare lowcompared with the frequencies used in actual transmission between stations. AThe essential principle underlying 4'this method is that of a'frequency vstepnp and a frequency step down of a plurality -of channels V-in common, in accordance wit addition and subtraction of frequencies `by modulation.

This general scheme involves at least two stages of modulation. Signaling currents of each of the multiplexv circuits entering Iinto the system are .impressed upon the car# rier currents of intermediate frequency and these carrler currents are collected upon a comme-n circuit at this intermediate frequency. The frequency of the group is'then stepped up to the transmission frequency by modulation with a carrier of the desired radio frequency. At the receiving end the process is reversed, the frequency being stepped down to the intermediate frequency level lby subtracting the original radio frequency and the superposed channels are then separated by suitable selective circuits at the receiving frequency. Each channel is then detected individually and becomes a circuit of ordinary signalingA frequency.

y By performing the selecting and separating actions at a relatively low carrier frequency, instead of at the radio transmission frequency, an important advantage arises in connection with the design of selective circuits. Fora o'iven investment in the selective circuits far greater discrimination as measured in terms of frequency intervals rather than percentage of frequency range,

can be obtained at low than at high frequecy.

A further advantage of a selective system of thls type resides in the fact that it facilitates the suppression of one of the two side bands of modulation and thereby permits of .reducing the width-of the band of frequencies required to transmit the actual signal variations themselves. By thus reducing t-he width of the signaling bands, the channels may :be more closely spaced and a larger number of channels obtained from the avail'- able frequency range.

A selective system of the type outlined above permits the communication channels to be placed so closely together in the radio frequency` range as to practically requiie the use of some frequency controlv system for maintaining a rigid frequency spacing between channels and'thereby preventing them from overlap-ping and interfering. The invention therefore contemplates generating a controlling frequency from which all other frequencies used in the system may be obtained. This may be done in several ways, for instance.l there might 'be transmitted from the sending sta-tion'a carrier frequency which is much more powerful than the side bands due to modulation, or the received carrierv wave may be selectively ampliied to the exclusion o-f the side bands. The first of these methods is not economical, whi-le the second method is impractical, by reason of the difficulty of 'selecting the carrier atthehigh frequencies used in radio transmission. It is, however, feasible t0 select out the received modulation.

i quency itself, so as to be enabled to employ it in demodulating received radio frequency channels.

Specifically it is proposed, therefore, to generate at the sending station a fundamental frequency and produce harmonics of thisfundamental frequency. The lower har' monics are then used as carriers for the first stepof modulation and a higher harmonic is used as the carrier for the second step of At the receiving stationv either of two things may be done: A fundamental 'frequency may be independently generated at the receiving station and harmonics may be produced from this fundamental, one of I the higher harmonics being used for the first step of frequency translation and lower harmonics used for the final step of frequency translation. By this scheme all of the channels at each station are definitely tied together, since they are related to a fundamental frequency, and it is a matter of relatively simple adjustment to maintain the fundamental frequency at the sending and receiv ing station the same. An valternative expedient is to detect from the received band of frequencies the fundamental frequency generated at the sending station and produce from this frequency harmonics which may be used in the steps of demodulation, as above described. This expedient not only definitely ties together the frequencies of the various channels at the sending and receiving station, but also rigidly relates the frequencies of corresponding channels at the two stations. l

The invention may now be more fully understood from the following detailed description when read in connection with the accompanyingdrawing, Figures 1 and 2 of which constitute circuit diagrams of a sending and receiving station, embodying one form of the invention, Figures 3 and 4 constitute diagrams of sending and receiving stations embodying another form of the invention, and Fig. 5 of which discloses apparatus for practicing a. modification of the invention.

Referring to Figure 1, AT designates a transmitting antenna arrangement of any ordinary construction, while L, designates a low frequency signaling circuit, signals from which. may be radiated from said antenna, after undergoing suitable frequency conversion. The signaling circuit L1 is associated, through a balanced transformer 11, with a transmitting channel TL while a corresponding receiving channel RL, is. connected to the midpoint-s of the transformer windings. The apparatus associated with the receiving channel RL, is not'illustrated, but may be understood from the corresponding receiving arrangement of a distant station, as illustrated in Figure 2. The channels TL, and RL, are rendered conjugate with respect to each other, by balancing the line L by means of an artificial line or network FL, in a well-known manner.

The channel TL, includes a modulator M, and a band filter T F,. The modulator M, may be of any well-known type, but is illustrated as a vacuum tube modulator which is supplied with carrier current through a tuned circuit 21. in a manner more fully hereinafter described. The band filter TF, is of the general type disclosedY in the U. S. patents to George A. Campbell, Nos. 1,227,113 and 1,227,114, issued May 22, 1917. This band filter is designed in accordance with the principles of said Campbell patent, to transmit the carrier frequency supplied to the modulator M, and one of the side bands resulting from modulation, but the other side band is preferably suppressed. The filter TF, connects the channel TL, to a common transmitting circuit TL.

The other channels, such as TL2 and TL3, corresponding to other signaling lines (not shown) are also connected to the common transmitting circuit TL through filters TF2 and TF3, respectively. The channels TL, and TL3 lalso include modulators M2 and M3, which are supplied with carrier currents of different frequencies, through tuned circuits such as 22 and 23. The filters TF 2 and TF are in'general similar to the filter TF but are designed to transmit the carrier frequency assigned to the channel, together with one of the side bands due to modulation, the other side band being preferably suppressed.

In order to provide the necessary carrier frequencies for the different channels, a source G is provided for generating a fundamental frequency. This source or generator may be of any type, such as, for instance, the well-known vacuum tube oscillator. The fundamental frequency generated may be any suitable frequency and for purposes of illustration is indicated on the drawing as being 5,000 cycles. Harmonic frequencies are obtained from this fundamental frequency, by means of a distorting tube arrangement DT. This distorting tube arrangement is of a well-known type and is fully set forth and described in the U. S. application of B. 1V. Kendall, Serial No. 139,530, filed Decemberl 29, 1916. The fundamental frequency and the various harmonies are supplied from the distorting tube DT to a common circuit 20, from which the desired carrier frequencies may be selected bv means of the tuned circuits 21, 22 and 23,

for application to the different modulators.

000A plus 5,000 cycles,

As illustrated, the fundamental frequency of 5,000 cycles is applied to the modulator M3 and the first harmonic of 10,000 cycles to the modulator M2 and the third harmonic of 15,000 cycles to the modulator M1.

As the several carrier frequencies used in the first step of modulation are not suiiiciently high for radio purposes, a common modulator M is provided for stepping up the severalbands of frequencies impressed upon the common transmitting circuit TL. `This modulator may be of any type, but is preferably a duplex vacuum tube modulator of the type illustrated in the U. S. application of John R. Carson, Serial No. 157,413,`filed March 26, 1917, issued as Patent No. 1,343,307, dated June 15, 1920.v The circuit rTL is associated with vthe modulator M through a transformer 24. In order to supply carrier currents of the desired radio frequency to the modulator M, one of the higher harmonics of the fundamental frequency may be taken from the circuit 20 and supplied to the modulator M, over the circuit 25. As

the carrier frequency desired for radio purposes may be quite high, and in the case illustrated is 500,000 cycles, which corresponds to the hundredth harmonic of the fundamental frequency, it is not practical to take this harmonic through the circuit 20 directly,

lconsequently a lower harmonic such as the tenth harmonic, which corresponds to a f requency of 50,000 cycles, is selected by means of a' tuned circuit 27 and is a plied to an amplifying and distorting' tube T, similar to the distorting tube DT, already referred to. -This distorting tube produces amplified kn harmonics of the basic frequency 50,000 cycles, and the tenth harmonic corresponding to 500,000 cycles may 'then be selected bymeans of the tuned circuit 28. A power amplifier PA is provided for amplifying vthe selected harmonic which is then transmitted over the circuit 25 to the modulator M.

The modulator M is associated with the antenna AT, through a transformer 29. As a result of the second step of lmodulation, neglecting for the moment the side bands developed i frequencies will appear inthe output circuit of the modulator M, corresponding to 500,-

500,000 plus 10.000 cycles and 500,000 plus 15,000 cycles. Frequencies of 500,000 minus 5,000 cycles, 500,- 000 minus 10,000 cycles and 500,000 minus .15,000 cycles will also appear. In' between n the modulators M1, M2` andMa,

demo 000 cycles. If, however, it is desired to radiate one side band in preference to the other, the antenna AT will be tuned to the average frequency of the side band to be radiated. If, for instance, the upper side band is to be radiated, the antenna should be tunedto about 510,000 cycles.

The receiving apparatus is illustrated in Figure 2, in which AR designates a receiving antenna at adistant station and L1 a low frequency signaling circuit upon which the radio signals received by the antenna R may be impressed, after having undergone a suitable translation- A transmitting channel TLl is associated with the line L1l through a transformer 11'.. The apparatus associated with this channel is not illustrated, but it will be understood to be similar to that illustrated in Figure 1. A receiving channel RL1 isA connected to the midpoints of the windings of, the transformer 11 and the transmitting and receiving channels are rendered conjugate by balancing the line L1 with an artificial line or network N1. A

The receiving channel RLl, together with the receiving channels RL2 and RL3 corresponding to other low frequency signaling lines, are connected to a common .recelving circuit RL, through band filters RF1 RF2 andRF3.' These filters may be identical in construction with the corresponding filters TFi, .TF2 and TF 3 of Figure 1. The receiving channels RLI, RL,J and RL3 also include detectors D1, D2 and D3. These detectors may be of any wellown type, but are preferably vacuum tube detectors. If desired, an amplifier such as RA1 may be included in the output circuit of each. detector, for amplifying the detected low frequency signals.

This amplifier may be of any desired type,

but is preferably a vacuum tube amplifier.

coupled through a transformer 30 with a local oscillating circuit 31, tuned to Athe -same frequency. The` circuit 31 is connected throu h atransformer 32 with a common 115 dulator or detector D. This detectorA may be of anywell-known type, butis preferably a balanced duplex vacuum tube detector of the general type illustrated in the U. S. application lof John R. Carson, Serial No. 157,414, filed March26, 1917, issued as Patent No. 1,343,308, dated June 15, 1920. The function of this detector is to step down the received bandof radio frequenciesv to apoint in the frequency spectrum corresponding to the first stage of modulation at the distant sending station. This result is accomplished by beatingv the received radio frequencies with a locall supplied carrier frequency corresponding the frequency used in the last step of modulation at the distant sending station. In the case illustrat'ed, this frequency is 500,000 cycles.

The detector D is associated with the common receiving circuit. RL, through a transformer 33 and from this circuit the stepped down radio frequencies are supplied to the detectors D1, D2 and D3, through the filters RF1, BF2, and RF3. In order that these detectors may efficiently detect the low frequency signals from the band supplied thereto, it is desirable that the selected bands should be beaten With locally supplied liomodyne frequencies corresponding to the carrier frequencies used in the first step of modulation at the distant sending station.

In order to supply these homodyne frequencies, as well as the beating frequency of 500,000 cycles to be used in the first step of demodulation, an ordinary vacuum tube detector D is associated through a transformer 34 with the oscillating circuitI 31. The Various frequencies in the, received band, upon being impressed upon the detector D1 react with each other in aA Well-known manner to produce in the output circuit of the detector frequencies corresponding to the sums and -differences of the various frequencies. The high frequencies are of no utility and prominent among the low frequencies will be the fundamental frequency of 5,000 cycles. This will be apparent when it is considered that the three basic frequencies radiated from the distant sending station are 505,000, 510,000 and 515,000 cycles. The common difference between these frequencies is therefore 5,000 cycles. By means of a tuned circuit 35, associated with the detector D1, the 5,000 cycle frequency may be selected from the various other frequencies resulting from the operation and detection. An amplifier A is provided for amplifying the selected fundamental frequency. A distorting tube arrangement DT, similar to the distorting tube DT at the sending station, is provided for the purpose of producing harmonics of the fundamental frequency. The fundamental frequency, together With its various harmonics are supplied from the distorting tube DT to the circuit 36, from which the desired homodyne frequencies may be supplied to the detectors D1, D2 and D, through tuned circuits 37, 38 and 39, respectively. In the case illustrated, these circuits will be tuned to 15,000 cycles, 10,000 cycles and 5,000 cycles respectively.

In order to provide the beating frequency for the detector D, a circuit 40 tuned to one of the higher harmonics of the fundamental frequency, such as the tenth harmonic, is associated with the circuit 36. The frequency of. 50,000 cycles, selected by this harmonic is then impressed upon a. second distorting tube arrangement DT, similar in character to the distorting tube arrangement DT at the distant sending station. This arrangement functions to produce harmonics of the 50,000 cycle frequency, of which the tenth harmonic corresponding to a frequency of 500,000 cycles may be selected by a tuned circuit 41. This frequency is then supplied to the detector D over a circuit 42.

Further details of the apparatus will be clear from the description of the operation, which is as follows: Low frequency signals incoming from circuits such as L1, are impressed upon the transmitting channels TL1, TL2 and TL3 and react in the modulators M1, M2 and M3 with the carrier frequencies of 15,000, 10,000 and 5,000 cycles, to produce in theoutput circuit of the modulators carrier frequencies corresponding to the channels, together with side bands due to modulation. The filters TF1, TF2 and TF3 then transmit one side band and the carrier frequency of each channel and suppress the other side band` the transmitted side bands and carrier frequencies being impressed upon the common transmitting circuit TL. By suppressing one side band for each channel, the channels may be spaced much closer together in the frequency spectrum.

The composite band of frequencies now appearing in the transmitting circuit TL are impressed through the transformer 24 upon the modulator M, to modulate the radio frequency of 500,000 cycles. This operation results in stepping the composite band appearing in thecircuit TL up to a double band in the neighborhood of 500,000 cycles. Both side bands of this double band may be radiated, if desired, or the radiation of one side band may be minimized, depending upon the timing of the radiating antenna T, as already described. The radio frequency carrier current of 500,000 cycles is not transmitted as it is suppressed by the modulator M.

The frequencies radiated from the antenna AT are absorbed by the antenna' AR at the distant receiving station of Figure 2 and are transmitted through the transformer 30 to the detectors Dy and D. The` detector D produces in its output circuit the fundament-alv frequency of 5,000 cycles, in the manner already described. This frequency is selected by means of a tuned circuit arrangement 35 and after being amplified by the amplifier A is passed through the distorting tube arrangement D2', 'thereby producing in the circuit 36 the fundamental frequency of 5,000 cycles, together with its various harmonics. The fundamental frequency and the second and third harmonics are supplied to the detectors D3, D2 and D1, respectively, through the tuned circuits 39, 38 and 37, respectively. The tenth harmonic (50.000 cycles) is supplied through the tuned circuit 40 to the distortcwhile the sums of the frequencies will also appear in the output circuit of the detector.

These may be neglected, as they will be tuned out. -As a result of this operation the same composite band of frequencies ap-- pears in the circuit RL as was superposed upon the 'transmitting circuit TL at the distant sending station, as a result of the first step of modulation.

The several bands making up this composite band will be selected by the filters RF RF2 and RF3 and impressed upon the detectors D1, D2 and D3. The individual bands will react in the detectors with the corresponding homodyne frequencies, 15,000 cycles, 10,000 cycles and 5,000 cycles, respectively, so that low frequency currents corresponding to the original low frequency signals will appear in the output circuits of the detectors and after amplification by amplifiers such as the amplifier RAl, will bel transmitted to the signaling lines, such as the line LL.

A modified arrangement is diclosed in Figures 3 and 4, in which the feature of tying together corresponding channels at the'sending and receivingstations is eliminated, the frequency f-spacing' Ybetween` the`v channels being fixed, however, by developing the carrier frequencies at each station from a fundamental frequency. Referring to Figure 3, which illustrates the transmitting arrangement, the apparatus is in general similar to that of Figure 1. For purposes of illustration, however, the frequencies used are indicated as being different from those used in Figure 1. Thus the frequency generated by the oscillator G is indicated as 4,000 cycles and the first and second harmonics supplied to 'the transmitting channels TL2 and TL1 are therefore 8,000 and 12,000 cycles respectively. Since, in accordance with this scheme, the fundamental frequency of 4,000 cycleswill not be detected at the receiving station, it is unnecessary to transmit the vcarrier frequencies involved in the first modulation. The modulators M1, Mz-and M3 are there, fore of the duplex vacuum tube type disclosed in the U. S. application of John R. Carson, aSerial No. 157,413, already 'referred to. Y v

In order-to supply the proper radio carrier frequency to the common modulator M, in this instance the twenty fifth harmonic is selected by a tuned circuit 27 and this jlLa band filter BF frequency (100,000 cycles) is supplied to the modulator M over the circuit 2'5. Preferablya power amplifier PA should be provided in the circuit 25 for amplifying this harmonic.

The band filters TF1, TF, and TF3 may be arranged to suppress one side band, preferably the lower band. The basic carrier frequency of each channel will be suppressed by the modulator. If, therefore, it be assumed that the signalstransmitted over the low frequency lines are ordinary telephonic currents ranging from Zero to 2,000 cycles in frequency, the .band transmitted by the filter TF1 will be 12,000 to 14,000 cycles, that transmitted by the filter TF 2 will' be 8,000 to 10,000 cycles and that transmitted by the filter TF3 will be 4,000 to 6,000 cycles, consequently the composite band supplied to the common transmitting circuit TL will be 10,000 cycles in width and extend from 4,000 cycles to 14,000 cycles. The modulator M, after modulating the radio frequency of 100,000 cycles in accordance with this composite band, will produce in its output circuit two side bands extending from 104,000 to 114,000 cycles and from 96,000 down to 86,000 cycles. The carrier frequency of 100,000v cycles will be suppressed by the'modulator'M, as before, and the band filter BF may be arranged to suppress the lower of the two bands, so that frequencies from 104,000 to 114,000 cycles will be radiated.

At the receiving station shown in Figure similar in all respects to the bandfilter BF will be provided between the antenna AR and the demodulator D. As the fundamental frequency is not to be detected at the receiving station, the detector corresponding to the detector D of Figure 2 will be omitted. A generator G is provided instead, for generating the fundamental frequency. This generator may beY of any Well-known type, but 1s preferably 110 a vacuum tube oscillator similar to the oscillator G of Figure 3. This oscillator is provided, however, with suitable means schematically indicated at 43, for adjusting its frequency in order to maintain the frevquency generated in synchronism with that generate-d by the oscillator Gr at the sending station. The frequency adjusting means may control capacity or inductance in the frequency determining circuit of the oscillator. A harmonic producer DT, similar to that shown in Figure 3, is provided for producing harmonics of the fundamental frequency (4,000 cycles). The first, second and third harmonics are supplied to the detectors D D2 and D1, respectively, through tuned circuits 39, 38 and 37.

The detectors D1, D2 and D3 are herein illustrated as being duplex vacuum tubedetectors of the general type disclosed U.- S. application of John R. Carson, Serial Nofll ,414, already referred to. The filters RF1, RF2 and RF3 will be in all respects similar to the ycorresponding lte'rs TF1, TF2 and TF3 at the sending station, and will transmit the same band of frequencies.

In order to supply the p-ro-per homodyne frequency to the detector D for the first step of demodulation, a tuned circuit 40 is provided to select the twenty fifth harmonic from the circuit 36 and impress it over the circuit 42 upon the demodulator D. A power amplifier PF may beincluded in the circuit 42, lfor amplifyingthe homodyne frequency which should' be of large powei compared with the received frequency.

Further details of the. apparatus will now be clear yfrom a description of the operation, which isas follows: Low frequency signals admitted to the channels TL1, TL2 and TL are impressed upon the modulators M1, M2 and M .thereby modulatingr carrier frequencies of 12,000 cycles, ,8,000 cycles and 4,000 cycles respectively, which are supplied from the harmonic producer DT. rIhe modulators M1', M2 and M3 suppress the carrier :frequency and the filters TF1, TF2 and TFJ suppress the lower side band of each channel, while transmitting the upper side bands. The several transmitted side bands are collectedin a common transmitting circuity TL and impressed upon the common modulator M, which modulates the frequency of 100,000 cycles supplied overy the circuit 25, in accordance with the frequencies^contained in the composite side band transmitted to the circuit TL. The carrier frequency of 100,000 cycles is suppressed by the modulator Muand the lower side band resulting from the second step of demodulation is suppressed by the filter BF, while the upper band ranging from 104,000 t0 114,000 cycles is transmitted to the antenna for radiation to the distant receiving station.

At the dist-ant receiving station the generator G is adjusted until it generates the same basic frequency as the generator Gr at the sending station. The received band of `radio frequencies absorbed by the antenna AR is passed through the filter BF to the demodulator D. The frequency of 100,000 cycles `impressed upon the demodulator fiom the circuit 42 reacts with the received band of frequencies to produce sum and difference frequencies in its output circuit. The high frequencies corresponding to the sum of the modulating and modulated frequencies are of no utility and will be suppressed by the band filters, qwing to their high frequency. The band of frequencies corresponding to the difference between the modulating and modulated frequencies will be a composite band of frequencies ranging from 4,000 to 14,000 cycles. The individual signal bands making up this composite band will be selected by means of the filters RF1, RF2 and RF,s into corresponding receiving-channels RL1, RL2 and RL3 and impressed upon the detectors D1, D2 and D11, respectively. These bands, it will be noted, correspond to the bands resulting from the first step of modulation at the distant sending station and by reacting with the carrier frequencies as,-A signed to the channels will result in the production of low frequency signaling currents in the output circuits of the detectors corresponding to the signaling currents originating at the sending station.

lt will be observed that since all of the carrier frequencies involved at either the sending or receiving'station are multiples of a fundamental frequency, the. frequency spacing of the channels at either station and the correspondence of the frequency of similar channels at the two stationsy may be main-v tained fixed by the simple expedient of maintaining a proper adjustment of the oscillator producing the fundamental frequency at each station.

As has already been pointed out, one of advantage, namely, a great saving in the amount of power which it is necessary to radiatein order to obtain a given signal at the receiving station. For example, it may be shown that where two steps of modulation are employed and only a single channel system is provided, it will be necessary 'to radiate, under certain circumstances, 19

times as much energy where the carrier and both side bands are transmitted as where but a single side band is transmitted. In the case of three channels, it may. be shown that, under certain circumstances, 153 times as much energy will be radiated where the 'the advantages of the arrangement of applicarrier and both side bands are transmitted v In order to understand this 'more clearly, let it be assumed that there are tobel two steps of modulation, and that in ythe first step of modulation the amplitude of each side frequency where the signal ,is a simple singing note, is a. If we have. complete modulation, that is, a modulation in which lwhen the signal wave is positive the instantaneous amplitude of the carrier is twice the normal amplitude and when the Wave is negative theinstantaneous amplitude is Zero, the carrier employed in the first step of modulation mustl have an amplitude `of 2a. This follows at once when it isconsidered that the two side waves of amplitude a, when added to each other, must be equal lin amplitude and opposite insign to the carrier. Consequently, we will ,have in the output of the modulator a carrier of amplitude 2a, an upper side. frequency of amplitude a, and a lower-side frequency of amplitude a. ySince the energy is roportional to the square of the amplitu e, the total energy represented -by these three componentslwill be proportional to 4a? in the case of the carrier component and 0:2 in the case of each side frequency component, making a totalenergy proportional to 60,2. If only one side frequency is radiated and the carrier and,A other side frequency be suppressed at this stage, the energy transmitted will be proportional to a2.

If now we consider that all three components are passed on to the next -step of modulation and are used to modulate another carrier, it will be seen that the output of the modulator at the second stage will have a component corresponding to the second stage carrier, a lower side band including the components corresponding to the first stage carrier and each of the side frequencies resulting from the first step of modulation, and an upper side band also containing components corresponding to the first stage carrier and each of the side frequencies resulting from the rst step of modulation. In other'words, there will be seven components. Now let us suppose that at a given instant `all of the side components are in phase and ofthe same polarity, and that their amplitude has not been increased over that which existed in the firest stage of moddulation. If we add the amplitudes of the siX components other than the carirer, the total amplitude will be 8a, so that in order to obtain complete modulation, the second stage carrier would have to have an amplitude of 8a. If all of the energy is to be radiated, the energy corresponding to the second stage carrier would be proportional to 64a2, that corresponding to the three frequencies in the lower side band to 60:2, and that corresponding'to the three Ifrequencies in the upper side band would also be 6a?, making a total of 76a? If onl one side frequency had been transmitted a ter the first stage of modulation and thisside frequency wouldv have an amplitude of a, We would have at the second stage only a carrier frequency and upper and lower side frequencies corresponding to the side frequency transmitted from the first stage. If we suppress the carrier and one of these side frequencies, only a side frequency corresponding to the original signal andhaving an amplitude a, would be present, and the energy radiated would be proportional to a2. The signal produced in` this case at the receiving sta-V tionwould be only L as great as that proall of the frequencies are radiated, it would f only be necessary to radiate ,lg of the power required in the latter case.y This shows at .once that an immense saving in power results from the suppression of the carrier at all stages of modulation in a plural modulation system, and a smaller but an appreciable savingresults from the suppression of one of the side bands. y

Considering now the case where three channels are transmitted and two steps of modulation are provided, it will be clear that in the output of the second stage there will be a component corresponding to the second stage carrier, a lower side band including three components corresponding to the three first stage carriers and two fre quencies associated with each of these three frequencies, one frequency of each pair corresponding to one side frequency at the first stage and the other frequency ofthe pairv correspondingl to the other side frequency. There w1ll also be an upper side band containing corresponding components, so that altogether there will be nineteen components. Assuming complete modulation in the first stage, each component'corresponding to a first stage carrier will have an amplitude of twice the component corresponding to the signal. The sum of the amplitudes of all of the carriers in the two side bands of the second stage, there beingV altogether six carriers, will be 12a; The amplitudes of the twelve side frequencies involved will also total 12a. If, at a given instant all of these frequencies should be in phase and ofthe same sign, the total amplitude would be 24a and it would be necessary that the second stage carrier have an ampli nde of 24a in order to have complete mo lation. In practice, of course,

the probability that all of these frequencies plitude considerably greater than the amplitude 8a required in the case of the single channel, and for perfect operation, its amplitude, as above stated, should be 24a. Assuming` that the amplitude be taken as the latter figure, the energy radiated in case all components are transmitted, would be proportional to 576e? in the case of the sec ond stage carrier, 18a'Z in the case of the :nine frequencies in the lower side band, and 18502 in the case of the nine frequencies in the upper side band,making a total of 612a?. lf we diiide 612 by l to take into account the fact that where both side bands are radiated, the signal will be 4 times as strong in the case of double demodulation as where but a single side band is radiated, we sce that it will be necessary to radiate 153 times as much power where all components are transmitted as would be the case where but a single side band is'transmitted for each stage. Practically, \as above stated, this ligure might be reduced by using a carrier at the second stage of less amplitude than 24a and putting up with a certain amount of distortion. It is safe to say, however` that it would be necessary to radiate much more than 19 times the energy in the one case than in the other, and for perfection of operation, it would be necessary to radiate 153 times as much energy.

The suppression of the carrier particularly, and to a lesser extent the suppression of one of the side bands for each stage of modulation, is then a very important factor from the standpoint of energy saving as well as from the standpoint of close spacing of the channels. This is especially important where, as is usually the case, the energy is to be amplified before radiation, as it will be necessary to provide an amplifier which would handle 153 times as much energy in the one case as in the other under the conditions considered in connection with three channels, It is important to note that each time a channel is added, the energy radiated increases by an amount greater than a simple multiplication of the energy required for a single channel by the total number of channels. v

As a further development of the method involved in this invention the successive steps of demodulation may be increased as will be clear from the following concrete statement of the method illustrated in Fig. 5. Let it be assumed that at a given station it is desired to select all frequencies lying between 600,000 and 610,000 cycles and to exclude all frequencies lying outside of that range. If the selection were to be made by the ordinary methods of discrimination now practised in the'radio art this would involve excluding such frequencies as 599,500 cycles 610,500 cycles, which would be practically impossible. It is therefore proposed to impress the various received frequencies upon acircuit tuned sharply to about 605,000 cycles, which, it will be noted, is practically the mean of the limiting frequencies of the desired band. Since the desired band is relatively narrow asv compared with the range of frequencies extending from zero up to the desired frequency, this step would give practically uniform reception for the frequencies between 600,000 and 610,000 cycles and decreasing reception for other frequencies. Such a circuit might, for example, give only ten per cent reception of current of 550,000 cycles.

The next step of the method involves beating the various frequencies with a. locally supplied frequency for the purp-ose of stepping down the frequency of the desired bland. The locally supplied frequency should be, chosen as close as possible to the desired band and should be sufficiently below said. band, so that frequencies resulting from the reaction of the local frequency with the undesired frequencies will, if they fall within the limits of the band resulting from the reaction of the desired frequencies with the locally supplied frequency, be practically negligible amplitude. For instance, assuming that a frequency of 550,000 cycles be used for beating the received frequencies, this would'reduce the desired frequenc-ies to a band between 50,000 and 60,000 cycles, which would be transmitted to the next circuit through a band filter. Although frequencies between 50,000 cycles and 60,000 cycles would also be produced by t-he reaction of the locally supplied 550,000 cycle currents with received currents in the range between 500.000 and L190,000 cycles, since the last mentioned band of frequencies is quite distant from the frequency to which the first circuit is tuned, the resultant frequencies between 50,000 and 60,000 cycles will be of very small amplitude as compared with frequencies within the same range resultin from the stepping down of the desired band of frequencies.

After stepping down the desired band of frequencies and filtering in the manner just described, the resultant band may be again stepped down by beating with locally sup plied currents of a lower frequency vwhich should be chosen with regard to the same considerations as stated in connection with the first step of lowering frequency. 1f it be assumed for purposes of illustration, that the second locally supplied frequency be. 40,000 cycles, its reaction with the selected band would reduce the frequencies of said band to the range between 10,000 and l20,000 cycles. This rangeA would then again be passed through a band filter.

The process may be continued as many times as may be necessary to pick out the high frequencies desired. 1n the case supposed, probably athird stepping down of the frequency would be sufficient. For instance, if the band between 10,000 and 20,000 cycles is beaten with a locally supplied frequency of 10,000 cycles, the currents will then lie in the frequency range from zero to 10,000 cycles and interfering currents would be eliminated. v

A form of apparatus for carrying out the W`method is illustrated in Fig. 5. Referring to v 11 and a local beating frequency maybe impressed upon said input circuit by means of a connection 12 leading to a generator Gr,

which supplies a. frequency of 550,000 cycles,

for instance. A demodulator of this type, as is well known, produces in its output circuit frequencies corresponding to the sums and differences of the frequencies applied to its input circuit, consequently a frequency band between 50,000 cycles and 60,000 cycles will appear in its output circuit, corresponding to, the reaction of the locally supplied frequency of 550,000 cycles with the desired received frequency band ranging from 600,000 to 610,000 cycles.

A band lilter F1 is connected with the output circuit of the demodulator D1. This filter may be -of the type disclosed in the U. S. patents to -George A. Campbell, Nos. 1,227,113 and 1,227,114, issued May 22, 1917. The filter* should be designed in accordance with the principles of said Campbell patents, so as to freely transmit frequencies lying between 50,000 and 60,000 cycles, while suppressing frequencies lying without these limits.

A second demodulator D2, similar in type to the demodulator D1, has its input circuit associated with the filter F1. Low beating oscillations may be supplied over a circuit 13 from a generator G2 to the input circuit of the demodulator D2. As a result of these connections the' output circuit of the demodulator will supply a frequency band lying between 10,000 cycles and 20,000 cycles, said band corresponding to the band passed vby the filter F1 stepped down to a lower position .in the frequency spectrum. A filter F2, in general similar to the filter F1, but having a free transmission range between 10,000 and 20,000 cycles, is associated with the output .circuit of the demodulator D, for selecting this band of frequencies.

A third demodulator D3 has its input circuit associated with the filter F2 and is supplied over a circuit 14 with local oscillations from a generator G3. The frequency of these oscillations may be, for instance, 10,000 cycles. The action of this demodulator will step the band passed by the filter F2 down to a range from zero to 10,000 cycles in the frequency spectrum. A filter F3 of the Campbell type maybe provided for selectlng this band to the exclusion. of the other,-

and.

The desired band of frequencies now appears in the circuit 15 as a band lying between zero and 10,000 cycles. The desired individual frequencies or groups of frequencies may be selected from this band by tuned circuits such as 16, 17 and 18 and may be impressed upon the circuits of detectors such as D4, D5 and D6 of any well known character. These detectors function to detect the selectedfrequencies so that they may actuate the receiving devices such as R4, R5 and R, of any well known type.

Obviously the converse of the method just described may be employed at'the sending stations so that a larger number of steps of modulations may be used. Furthermore it will be understood that thefrequencies hereinbefore described are merely illustrative and may be varied in practice as conditions ma 7 require.

t will also be understood that the details of the methods herein set forth may be varied and that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated, Without departing from the spirit of the invention as defined in the fol-lowing claims.

What is claimed is:

1. The method of radio signaling, which consists in receiving a band of radio frequencies, detecting from said band a fundamental frequency, producing harmonics of said fundamental frequency, selecting one o-f the higher harmonics, beating the band of radio frequencies with the selected harmonic, thereby producing a band of frequencies equal to the difference in frequency between said harmonic and the radio band, selectingl frequencies from the resultant band into receiving circuits and detecting from the selected lfrequencies low frequency signals.

2. The method of radio signaling, which consists in generating a fundamental fre- 1'20 quency, producing harmonics thereof, modulating'certain of the harmonics in accordance with low frequency signals, modulating a higher harmonic in accordance with the resultant modulated frequencies, thereby producing a band of radio frequencies, transmitting said band to a receiving station, detecting from said band said fundamental frequency, producing harmonics of said fundamental frequency, beating the re- VJtion, producing harmonics of said fundamental frequency at said station, selecting at said station the modulated harmonics produced at the transmitting station, and beating the selected frequencies with corresponding harmonics, thereby detecting low frequency signals.

4. The method of radio signaling, which consists in generating at a transmitting station a fundamental frequency, producing harmonics of lsaid frequency, modulating certain of said harmonicsV in accordance with low frequency signals, modulating another harmonic in accordance with the resultant modulated frequencies, thereby producing a band of radio frequencies for transmission, receiving said -band at a receiving station, producing at said station a frequency corresponding to said fundamental frequency, producing harmonics of said frequency, beating the received band with one of the higher harmonics, thereby stepping the band down in the frequency spectrum, selecting frequencies from the stepped down band, and beating the selected frequencies with lower harmonics, thereby detecting low frequency signals.

5. The method of radio signaling, which consists in modulating a plurality of closely spaced carrier frequencies in accordance with low frequency signals, suppressing one side band of each of the resultant modulated bands so as to prevent interference with the unsuppressed band of the adjacent channel, modulating another frequency with each o f the unsuppressed bands, suppressing the lower side band of the resulting band, transmitting the unsuppressed band, receiving said band' at a distant receiving station, beating the received band with suitable frequency, in order to lower the band in the frequency spectrum, selecting from the band thus stepped down, bands corresponding to the side bands which were not suppressed at the transmitting station, and detecting from said bands low frequency signals. y

6. In a radio signaling system, means to receive a band of radio frequencies, a common demodulator upon which said band may be impressed, means to supply a beating frequency torsaid band, whereby thereceived band be stepped down infrequency, means to select from the band thus stepped down a plurality of smaller bands, a demodulator for each of the selected bands, means to supply a beating frequency to each of said bands, whereby low frequency signals may be detected from the selected bands.

7. In a radio signaling system, means to generate a fundamental frequency, meansto produce harmonics of said fundamental frequency, means to vmodulate certain of said harmonics in-accordance with low frequency signals, means to modulate another harmonic by means of the frequencies resulting from the first modulation, and means to transmit the band of frequencies resulting from the second modulation.

8. In a radio signaling system, means to receive a band of radio frequencies, means to produce a fundamental frequency, means to set up harmonics of said fundamental frequency, means tovbeat the received band of radio frequencies by one of said harmonics, thereby stepping the band down in the frequency spectrum, means to select smaller bands from the band thus stepped down, and means to beat the selected bands with other of said harmonics, thereby detecting low frequency signals.

9. In a radio signaling system, means' to generate a fundamental frequency at a transmitting station, means to produce harmonics of said fundamental frequency, means to modulate certain of said harmonics in accordance with low fre uency signals, means to modulate another armonie in accordance with the modulated harmonics, means to transmit to a distant receiving station the band of frequencies resultant from said last mentioned modulation, means to produce at said receiving station a fundamental frequency corresponding to the frequency generated at the sending station, means to produce harmonics of said frequency, means y to beat the received band of frequencies with one of said harmonics, thereby stepping the band down in the frequency spectrum, means to select smaller frequency bands from the band thus stepped down, and means to heat the selected bands with other of said har molnics, thereby detecting low frequency signa s. f

10. In a signaling system, means to generate at a sending station a fundamental frequency, means to produce harmonics thereof, means to modulate certain of said harmonics in accordance with low frequency signals, means to modulate another harmonicv in accordance with the modulated harmonics, means to transmit the resultant band of frequencies to a distant receiving station, means at said receiving station to detect from said band of frequencies said at said receiving station, means to beat the received band of frequencies with one of said harmonics, thereby stepping the band down bands of frequencies, and means to beat thel selected bands of frequencies with other of said harmonics, thereby detecting low frequenc signals.

11. he method of selecting frequencies from a band of radio frequencies which consists in beating the band of radio frequencies with another frequency so chosen as to produce aside band of -corresponding width and having a lower position in the frequency spectrum, selecting individual bands of the frequency band thus stepped down and beating each individual band with another frequency to detect signals therefrom.

l12. The method of selecting frequencies from a band of radio frequencies which consists in beating theband with another frequency to produce a side band of thesame width but `having a lower position in the frequency spectrum, and selecting from the band thus step-pcd down individual frequency bands without distorting frequencies in the individual bands.

13. The method of selecting frequencies from a band of radio frequencies which consists in beating the band with a-nothei` frequency to produce a. side band of the same width' but having a lower position in the frequency spectrum, selecting from the band thus stepped down individual frequency .bands` without distorting frequencies in the individual bands, and detecting signals from the individually selected bands.

14. The method yofreducing the energy transmitted in a system employing a plurality of steps of modulation, which consists in suppressing the carrier before transmission after each stepof modulation, and transmitting only the side bands.

15. The method of reducing the energy transmitted in a system employing a plurality of'steps of modulation, which consists in suppressing the carrier and one sidelband before transmission after each step of modulation, and transmitting only one side band for each step. l

16. The method of reducing the energy transmitted in a. multiplex system employing a plurality of steps of modulation, which consists in suppressing the carrier before transmission after each step of modulation for each channel, and transmitting only the side bands of each channel.

17.*'I`lie method of reducing the energy transmitted in a multiplex system employing a plurality of steps of modulation, which consists in suppressing the carrier and one side band before transmission after each step of modulation for each channel, and transmitting only one side band for each stepl of modulation in each channel.

18. In a signaling system employing a plurality of steps of modulation, means associated with each modulation stage for suppressing the carrier before `transmission to the next stage, whereby the energy appearing in the output of the last stage will be reduced.

19. In a signaling system employing a plurality of steps of modulation, means associat'ed with each modulation stage for suppressing the carrier frequency employed at that stage, and means associated with each stage of modulation for suppressing one side band resultingfrom the modulation at that stage, whereby only energy corresponding to one side b-and will appear in the last stage.

1 20. In a multiplex signaling system employing a plurality of stages of modulation, inea-ns associa-ted with each stage of modulation for each channel for suppressing the carrier frequency before ltransmission to the succeeding stage, whereby the energy in the output of the last stage of each channel will be reduced.

21. In a multiplex signaling system employing a plurality of stages of modulation,

means associated with each stage of modulation for eachchannel for suppressing the carrier frequency employed at that stage, and .means associated with each stage of modulation for each channel for sup-pressing one side band resulting from the modulation at that stage, whereby'only the energy corresponding to one side bland for eacli channel will appear in the output ofthe last stage.

22. yThe method of radio signaling, which consists in receiving a band of radio frequencies, obtaining from said band a con'- trolling frequency, producing. other fre-- quencies determined by said controlling frev quency, selecting one of the frequencies produced, beating the band of radio frequencies with the selected frequency, thereby producing a band of frequencies equal to the difference in frequency between the selected.

frequency and the radio band, selecting frequencies from the resultant band into receivingcircuits, and detecting from the resultant frequencies low frequency signals.

23. The method o fradio signaling, which consists in generating a controlling frequency, producing other frequencies determined by said controlling frequency, mod ulating certain ofthe produced` frequencies in accordance with low frequency signals, modulating another produced frequency in accordance with the resultant modulated frequencies, thereby producing a band of radio frequencies, transmitting said band to a receiving station, obtaining from said band the controlling frequency, producing other frequencies determined by said con- I trolling frequency corresponding-pto the frequencies produced from the controlling frequency at the transmitting station, beating the received band with one of the frequencies produced at the receiving station, thereby stepping down the band in frequency, selecting frequencies from said band and beating the selected frequencies with other of the frequencies produced at the receiving station, thereby detecting low frequency signals.

' 24. The method of radio signaling, Which consists in generating a controlling frequency, producing other frequencies determined thereby, modulating said produced frequencies in accordance with low frequency signals, thereby producing a band of frequencies, producing the same controlling frequency at a receiving station, producing other frequencies at the receiving station determined by said controlling frequency and corresponding to the frequencies produced from the controlling frequency at the transmitting station, selecting at the receiving station modulated frequencies transmitted from the transmitting station, and beating the selected modulated frequencies With the frequencies produced at the receiving station under the control of the controlling frequency, thereby detecting low frequency signals.

25. The method of radio signaling, which consists in generating at a transmitting station a controlling frequency, producing other frequencies at the transmitting station determined by said controlling frequency, modulating certain of said produced frequencies in accordance with low frequency signals, modulating another of said produced frequencies in accordance With the resultant modulated frequencies, thereby producing a band of radio frequencies for transmission, receiving said band at a receiving station, yproducing at said receiving station. a frequency corresponding. to the controlling frequency generated at the transmitting station, producing atthe receiving station other frequencies determined by said controlling frequency, beating the received band with one of said other frequencies, thereby stepping the band down in the frequency spectrum, selecting frequencies from the stepped-down band, and beating the selected frequencies With the remaining frequent-.es produced under the control of the controlling frequency, thereby detecting low frequency signals.

26. In a radio signaling system, means to generate a controlling frequency, means to produce otherA frequencies determined by said controlling frequency, means to 'modulate certain of said other frequencies in accordance With low frequency signals, means to modulate another of said frequencies by means of the frequencies resulting from the first modulation, and means to transmit the band of frequencies resulting from the second modulation.

27. In a radio signaling system, means to receive a band of radio frequencies, means to produce a controlling frequency, meansto obtain other frequencies determined by said controlling frequency, means to beat thereceived band of radio frequencies by one of said last mentioned frequencies, thereby stepping the band down in the frequency spectrum, meansto select smaller bands from the band thus stepped-down, and means to beat the selected bands With otherof the frequencies produced under the control of the controlling frequency, thereby detecting low frequency signals.

28. In a radio signaling system, means to generate a controlling frequency at a transmitting station, means to produce other frequencies determined by said controlling frequency, means to 'modulate certain of said other frequencies in accordance with low frequency signals, means to modulate another of said frequencies in accordance with the modulated frequencies, means to transmit to a distant receiving station a bandof frequencies resulting from the last mentioned modulation, means to produce at said receiving stationv a controlling frequency corresponding to the controlling frequency generated at the transmitting station, means to obtain other frequencies from said controlling frequency and corresponding to the frequencies -obtained from the controlling frequency at the sending station, means to beat the received band of frequencies with one of the frequencies produced at the receiving station, thereby stepping the band down in the frequency spectrum, nieans to select smaller frequency bands from the band thus stepped down, and means to beat the selected bands With other of the frequencies produced at the receiving station, thereby detecting low frequency signals.

29. In a radio signaling system, means to generate at 'a sending station a controlling frequency, means to produce other frequencies determined thereby, means to modulate certain o-f said other frequencies in accordance With low frequency signals, means to modulate another of said frequencies in accordance With the modulated frequencies, means to transmit the resultant band of frequencies to a distant receiving station, means at said receiving station to obtain from said band of frequencies said controlling frequency, means 'to produce at said receiving station other frequencies determined by said controlling frequency, means to beat the received band of frequencies with one of said other frequencies produced at the receiving station, thereby stepping the band down in the frequency spectrum, means to select from the band thus steppeddown smaller bands of frequencies, and

means tol beat the selected bands of frequencies with other of the frequencies promined by said controlling frequency, modulating certain of said other frequencies in accordance with low frequency signals, modulating another of said 4frequencies in accordancewith the resultant modulated frequencies, thereby producing a band 'of radio frequencies, and radiating the band of radio frequencies thus produced, l Y

32. The method of radio signaling, which consists inmodulating a plurality of carrier frequencies in accordance 'I with lovi7 frequency signals, suppressingthe carrier frequency component from each of the resultant frequency bands, modulating another frequency with each of the bands, and sup# pressing from the nal resultant band the component corresponding to the carrier frequency employed in'the'last mentioned step of modulation. u

33. The method of radio signaling, which l consists in modulating a plurality of carrie'r frequencies in accordance With low fre-k quency signals, suppressing one side band andthe carrier frequency component from each of the resultant modulated bands, modulating another carrier frequency with each of the unsuppressed bands, and suppressingv one of the `side bands and the carrier com? ponent fromv the band produced by the lastl mentioned step of modulation.`

V3ft. The method of radio signaling, which,l

consists in modulating a plurality of carrierV frequencies in accordance with'low fre-- quency signals, suppressing th'e carrier component from. each of the resultant modulated bands, modulating another carrier frequencyfof eachof the unsuppressed bands,

suppressing the carrier component corresponding to said last mentioned vcarrier frequencyfrom the band resulting from said last mentioned step of modulation, transmitting a band of frequencies including at least one of the side bands` involved in the last, step of modulation, receiving said band at afdist-ant receiving'station, beating the received band with asuitable frequency in order to lower the band in the, frequency spectrum, selecting from the band thus stepped-down bands corresponding to those resulting from the first step of modulation Vat the transmitting station, and detecting from said bands low frequency signals.

35. The method of radio signaling, which consists in modulating a plurality of carrier frequencies in accordance With loW .frequency signals, suppressing one side band and the carrier component from each of the resultantmodulated bands, modulating an other carrier-frequency with each'of thel unsuppressed bands, suppressing the carrier fre'quency'and one side band of the resultant band, transmitting the unsuppresse'd band, receiving said band at a distant receiving station, beating the received band with a suitable frequency in order to lower the band in the frequency spectrum, selecting from the band thus stepped-down bands correspond.- ing to the side'bands which Were not suppressed at the transmitting station, and dete'qtingfrom said bands low frequency signa s. A

36.' A radio receiving system comprising means for-receiving a plurality of bands of signaling oscillations, means for supplying responding to individual signals and supplying them to said detectors, respectively.

In testimony whereof, I have signedmy name to this specification this 29th day of September,1919.

LLOYD ESPENSCHIED. 

